The present invention relates to the improvements of a variable compression ratio mechanism for a reciprocating internal combustion engine.
In order to vary a compression ratio between the volume existing within the engine cylinder with the piston at bottom dead center (BDC) and the volume in the cylinder with the piston at top dead center (TDC) depending upon engine operating conditions such as engine speed and load, in recent years, there have been proposed and developed multiple-link type reciprocating piston engines. One such multiple-link type variable compression ratio mechanism has been disclosed in pages 706-711 of the issue for 1997 of the paper xe2x80x9cMTZ Motortechnische Zeitschrift 58, No. 11.xe2x80x9d The multiple-link type variable compression ratio mechanism disclosed in the paper xe2x80x9cMTZ Motortechnische Zeitschrift 58, No. 11xe2x80x9d is comprised of an upper link mechanically linked at one end to a piston pin, a lower link mechanically linked to both the upper link and a crankpin of an engine crankshaft, a control shaft arranged essentially parallel to the axis of the crankshaft and having an eccentric cam whose axis is eccentric to the axis of the control shaft, and a control link rockably or oscillatingly linked at one end onto the eccentric cam of the control shaft and linked at the other end to the lower end of the upper link. By way of rotary motion of the control shaft, the center of oscillating motion of the control link varies via the eccentric cam, and thus the distance between the piston pin and the crankpin also varies. In this manner, a compression ratio can be varied. In the reciprocating engine with such a multiple-link type variable compression ratio mechanism, the compression ratio is set at a relatively low value at high-load operation to avoid undesired engine knocking from occurring. Conversely, at part-load operation, the compression ratio is set at a relatively high value to enhance the combustion efficiency.
During operation of the reciprocating engine with the multiple-link type variable compression ratio mechanism, owing to a great piston combustion load (compression pressure) or inertial force a load acts upon the eccentric cam of the control shaft through the piston pin, the upper link and the control link. That is, owing to the piston combustion load, torque acts to rotate the control shaft in one rotational direction. Assuming that the magnitude of torque occurring due to piston combustion load is excessively great, a driving force needed to drive the control shaft to a desired angular position and to hold the same at the desired position has to be increased. This deteriorates an energy consumption rate of an energy source such as a motor. In other words, the energy source (i.e., the motor) has to be large-sized. Additionally, in order to withstand great torque occurring due to piston combustion load, the diameter of the control shaft has to be increased.
Depending on engine/vehicle operating conditions, switching from a part-load operating mode to a highload operating mode frequently occurs. During switching from part-load operation to high-load operation, the compression ratio is variably controlled to a low compression ratio suitable to high-load operation. Assuming that switching from high to low compression ratio is not rapid, engine knocking may occur undesirably. For the above reason, it is desirable to rapidly execute switching from high to low compression ratio.
Accordingly, it is an object of the invention to provide a variable compression ratio mechanism for a reciprocating internal combustion engine, which avoids or suppresses the maximum value of torque acting upon a control shaft owing to piston combustion load from excessively developing during operation of the engine.
It is another object of the invention to enhance the response to switch from a control-shaft angular position corresponding to a high compression ratio suitable for part-load operation to a control-shaft angular position corresponding to a low compression ratio suitable for high-load operation in a variable compression ratio mechanism for a reciprocating internal combustion engine.
In order to accomplish the aforementioned and other objects of the present invention, a variable compression ratio mechanism for a reciprocating internal combustion engine comprises a variable compression ratio mechanism for a reciprocating internal combustion engine including a piston moveable through a stroke in the engine and having a piston pin and a crankshaft changing reciprocating motion of the piston into rotating motion and having a crankpin, the variable compression ratio mechanism comprises a plurality of links mechanically linking the piston pin to the crankpin, a control shaft extending parallel to an axis of the crankshaft, an eccentric cam attached to the control shaft so that a center of the eccentric cam is eccentric to a center of the control shaft, a control link connected at a first end to one of the plurality of links and connected at a second end to the eccentric cam, an actuator that drives the control shaft within a predetermined controlled angular range and holds the control shaft at a desired angular position so that a compression ratio of the engine continuously reduces by driving the control shaft in a first rotational direction when at least one of engine speed and engine load changes from a first value to a second value higher than the first value and so that the compression ratio continuously increases by driving the control shaft in a second rotational direction opposite to the first rotational direction when the at least one of engine speed and engine load changes from the second value to the first value, and a distance from the center of the control shaft to a centerline of the control link passing through both a connecting point of the first end and a connecting point of the second end, measured with the piston near top dead center, being dimensioned so that the distance continuously decreases as the compression ratio decreases.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.